use FileHandle;

my $file = $ARGV[0];

my $fh_in;
if (defined($file)) {
   $fh_in = FileHandle->new($file, 'r')
   or die("Unable to open supplied argument: $!\n");
} else {
   $fh_in = *STDIN{IO};
}

while (<$fh_in>) {
   print($_); # do stuff
}